1. Field of the Invention
The present invention relates to a magnetic recording medium and a method of manufacturing the same. More particularly, the present invention relates to a magnetic recording medium having excellent electromagnetic characteristics, such as overwrite characteristics, and to a magnetic recording medium having high operational reliability provided with magnetic head tracking recesses capable of optical tracking of a magnetic head, and capable of effectively suppressing interference, due to the thickness of a magnetic layer, of reflected light to be used for servo tracking purposes.
2. Description of the Related Art
In recent years there has been a great need to increase the density of magnetic recording media and develop their capacity by increasing their linear recording density in order to realize expedient and inexpensive large capacity floppy disks. In the magnetic recording media manufactured by applying a magnetic coating material formed of a magnetic powder, binder resin, organic solvent and other necessary constituents onto a flexible substrate, such as a polyester film, and drying the material, setting the thickness of the magnetic layer as thin as possible is desirable to increase the liner recording density. When high density data tracks are laid out on a thin magnetic layer, smaller and larger capacity media are being developed due to an increase in track density by use of a technology which secures precise tracking by the placement of precise concentric markings and their detection as well as by an increase in liner recording density.
As seen in FIGS. 10 and 11, there are illustrated an enlarged sectional view and a plan view, respectively, of the essential portion of a conventional floppy disk of the above-mentioned type. As shown in these figures, a magnetic layer 101 is provided on the surface of a base film 100, with grooves 102 used for tracking servo formed in the magnetic layer 101 so as to extend in the direction of the rotation of a floppy disk, and a data track 103 formed between every two adjacent grooves 102. A tracking servo light beam 104, shown in FIG. 10, is projected onto the surface of such a floppy disk from a light-emitting element (not shown) provided in a magnetic recording and reproducing apparatus. Light 105 reflected by the surface of the floppy disk is received by each of light-receiving elements 106a, 106b, 106c and 106d separately provided in the magnetic recording and reproducing apparatus, as shown in FIG. 11. As the light intensity reflected on the data tracks 103 is different from that reflected on the grooves 102, the total output value of light-receiving elements 106a and 106b is constantly compared with that of the light-receiving elements 106c and 106d. Then, tracking servo of the magnetic head is performed so that both of the output values of these elements become equal.
However, in such a system it has been determined that relative tracking accuracy is rapidly degraded if the magnetic layer is under a certain thickness and the track density is increased over 1000 TPI. Furthermore, in studying the causes of this disadvantage, it has been found that the effect of optical noise on the tracking accuracy is remarkable when the track density is increased over 1000 TPI, which was not the case under 1000 TPI. It has also been determined that such an effect becomes remarkable especially over 2000 TPI.
In a conventional floppy disk having such magnetic head tracking recesses, the tendency has been to make the magnetic layer thinner as the recording density is increased. However, in such a medium the servo light projected on the surface of the magnetic layer is not 100% reflected on the surface of the magnetic layer. Part of the servo light is reflected inside the magnetic layer and on the surface of the substrate after it passed through the magnetic layer. Furthermore, the reflected light is likely to be affected by the varied magnetic layer thickness, and when the light reflected on the surface of the magnetic layer interferes with the other reflected light, the dispersion of the reflectivity becomes large, which causes a problem leading to improper magnetic-head tracking. Especially, when such a problem arises, there is a relationship between the magnetic layer thickness and the wavelength of the projected light. Optical noise is caused when any light other than the regular light reflected on the surface of the magnetic layer enters the servo sensors, and it has been found that the noise becomes remarkable when the thickness of the magnetic layer is under a certain level.
Also, it has been determined that among the media with the magnetic layer set under a certain thickness, when the disks have light transmittance of more than 5%, the optical noise becomes remarkably large and optical servo tracking can not be properly performed over 1000 TPI. Furthermore, in such media if the center line average roughness of the magnetic layer Ra is over one fiftieth of the wavelength of the projected light, optical servo tracking can not properly performed over 1000 TPI because the reflected light is scattered.